The invention relates to a plant for the electrolytic dissolution by oxidation of a metal comprising:
an electrolysis cell having a soluble anode based on the said metal to be dissolved and an insoluble cathode without an interposed membrane,
means for introducing, into the said cell, electrolyte bath to be enriched in ions of the said oxidized metal and means for removing, from the said cell, bath enriched in ions of the said oxidized metal, which means are suitable for keeping the said anode and the said cathode at least partially immersed in the said bath,
means for circulating an electric current between the said soluble anode and the said cathode, so as to dissolve the metal of the said anode in the said bath.
The invention also relates to a process for the continuous production of a solution of a metal which comprises the stages consisting in:
introducing, into an electrodissolution cell having a soluble anode based on the said metal and an insoluble cathode without an interposed membrane, electrolyte bath to be enriched in ions of the said metal, so as to keep the said anode and the said cathode at least partially immersed,
circulating an electrodissolution electric current between the said soluble anode and the said cathode, so as to dissolve the metal in the said bath by oxidation of the said anode,
removing, from the electrodissolution cell, bath enriched in ions of the said metal constituting the solution of the said metal.
The applications of the plants and of the processes of the abovementioned type relate to metals which dissolve poorly chemically; an important application relates to the enriching in stannous ions (Sn2+) of spent electrotinning solutions or the production of electrotinning solutions from virgin electrolyte.
The document EP 0 550 002 discloses a plant and a process of this type which are used to feed electrotinning solution to an electrodeposition cell having an insoluble anode and a material with a metal surface acting as cathode intended to be coated with tin; the electrodissolution cell of the plant for the production of electrotinning solution and the electrodeposition cell are connected in a closed loop or virtually closed loop, so that the enriched electrotinning solution is removed from the electrodissolution cell in order to be fed to the electrodeposition cell and so that, conversely, the electrotinning solution to be enriched is removed from the electrodeposition cell in order to be fed to the electrodissolution cell.
In order to avoid the disadvantages mentioned on page 2, lines 47 to 52 of this document, the electrodissolution plant does not comprise a membrane interposed between the soluble anode and the cathode.
According to the process disclosed in this document, as the electrodissolution plant does not comprise a membrane, when an electrodissolution electric current is circulated from the said soluble anode to the said cathode, a portion of the tin dissolved at the anode is deposited on the cathode, which is harmful to the overall dissolution yield of the plant for the production of electrotinning solution.
According to an essential characteristic of the electrodissolution process disclosed in this document, the rate of deposition of tin on the cathode is controlled at a level lower than the rate of dissolution of tin at the anode by reinforcing the reaction for the production of hydrogen on this cathode.
According to claim 8 of this document, one means for reinforcing the reaction for the production of hydrogen on this cathode consists in increasing the current density at this cathode, for example by decreasing the surface area of this cathode below that of the soluble anode.
According to this document, this plant and this process make it possible to enrich highly varied electrolyte baths, such as, for example, sulphuric acid baths, ferrostannate baths, methanesulphonate baths, cresol sulphonate baths, halide baths, fluosilicate baths and fluoborate baths.
As indicated on page 4, lines 46 to 50 of this document, it is preferable at the anode to maintain a current density below the critical current density at which oxygen begins to be formed, which makes it possible to prevent or at the very least to limit the formation of tin oxide (SnO) sludges.
As indicated on page 4, lines 50 to 52 of this document, the electrodissolution yield improves as the temperature of the bath increases and as the bath is stirred and homogeneous in composition.
An aim of the invention is to substantially improve the electrodeposition yields of the processes and plants of the abovementioned type.
To this end, a subject-matter of the invention is a plant for the electrolytic dissolution by oxidation of a metal comprising:
an electrolysis cell having a soluble anode based on the said metal to be dissolved and an insoluble cathode without an interposed membrane,
means for introducing, into the said cell, electrolyte bath to be enriched in ions of the said oxidized metal and means for removing, from the said cell, bath enriched in ions of the said oxidized metal, which means are suitable for keeping the said anode and the said cathode at least partially immersed in the said bath,
means for circulating an electric current between the said soluble anode and the said cathode, so as to dissolve the metal of the said anode in the said bath,
characterized in that it comprises means for maintaining an appropriate bath density gradient in the said cell so that, if D1 is the density of the bath in the vicinity of the cathode and if D2 is the density of the bath in the vicinity of the most active part of the anode, D2 greater than D1 and (D2xe2x88x92D1)xe2x89xa7100 g/l.
These means for maintaining a bath density gradient should thus not comprise a membrane.
In the case where the temperature of the bath is approximately homogeneous in the cell, this density difference corresponds essentially to a difference in concentration of oxidized metal ions; preferably, if C1 is the concentration of ions of this metal in the vicinity of the cathode and if C2 is the concentration of ions of this metal in the vicinity of the most active part of the anode, C1 less than  less than 10,000xc3x97C2; preferably, C1 remains below the concentration threshold beyond which a deposit of the said metal is formed on the cathode.
xe2x80x9cThe most active part of the anodesxe2x80x9d is defined as that combining the points of the anode where the current densities are highest and represent 90% of the current circulating between anode and cathode; this preciseness is important in the case where the anode is formed of granules of the said metal and where only a portion of the immersed granules contribute directly to the electrodissolution.
By virtue of the densitometric separation, the dissolved metal is not (or is only slightly) redeposited on the cathode and the overall dissolution yield is improved; the main advantage of the invention is that, as the concentration of the metal ions remains low in the vicinity of the cathode, the deposition of metal on the cathode is prevented or at least limited, which improves the overall electrodissolution yield of the plant.
The invention can also exhibit one or more of the following characteristics:
the means for maintaining the said density gradient comprise the positioning of the cathode in the bath at a level situated above the mean level of the most active part of the anode, the difference in level between that of the cathode and that of the most active part of the anode being adjusted in order to maintain the said density gradient.
In this configuration, the evolution of hydrogen does not disturb the bath region between the anode and the cathode, which facilitates the maintenance of the said density gradient.
the means for maintaining the said density gradient comprise means for maintaining an appropriate temperature gradient in the bath so that, if T1 is the temperature of the bath in the vicinity of the cathode and if T2 is the temperature of the bath in the vicinity of the most active part of the anode, T1 greater than T2 and the difference (T1xe2x88x92T2) is adjusted in order to maintain the said density gradient; preferably, (T1xe2x88x92T2) greater than 15xc2x0 C.
The bath density gradient is thus reinforced by thermal means; preferably, these means comprise means for cooling the bath in the vicinity of the most active part of the anode.
the means for maintaining the said density gradient comprise:
the appropriate positioning of the means for introducing bath to be enriched in order to introduce the bath into the said electrodissolution cell above the level of the cathode,
the appropriate positioning of the means for removing enriched bath in order to remove the bath below the level of the most active part of the anode.
These positionings also contribute to the maintenance of the bath density gradient in the cell; the bath to be enriched can be xe2x80x9cspentxe2x80x9d bath weakly concentrated in oxidized metal and/or xe2x80x9cfreshxe2x80x9d bath not comprising oxidized metal.
the said cell comprises a cathode compartment separated from the remainder of the bath by a partition which passes through the surface of the bath and extends downwards to the level of the cathode.
The advantage is then that the evolution of hydrogen is channelled into this compartment and does not disturb the entire bath, which makes it possible to achieve the best maintenance of the density gradient.
the means for introducing bath to be enriched are appropriate for introducing, into the cathode compartment, xe2x80x9cfreshxe2x80x9d bath not comprising ions of the oxidized metal.
The term xe2x80x9cfreshxe2x80x9d bath not comprising ions of the oxidized metalxe2x80x9d is understood to mean xe2x80x9cvirginxe2x80x9d electrolyte in which no addition of ions of the oxidized metal has been carried out.
As bath not comprising metal ions is then introduced in the vicinity of the cathode, the concentration of metal ions in the vicinity of the cathode is virtually zero, which decreases in proportion the deposition of metal on the cathode and improves the overall electrodissolution yield of the plant.
the said cell comprises at least one anode compartment separated from the remainder of the bath by a partition which passes through the surface of the bath and extends downwards until above the level of the most active part of the anode.
the means for introducing bath to be enriched are appropriate for introducing bath into the anode compartment.
In this configuration, the introduction of bath does not disturb the bath region between the anode and the cathode, which facilitates the maintenance of the said density gradient.
the said anode is formed essentially of granules of the said metal.
Advantages: ease of replacement, high contact surface area with the electrolyte, resulting in a fall in the current density, which decreases the risks of evolution of oxygen and of formation of sludges.
Another subject-matter of the invention is a process for the production of a solution of a metal comprising stages consisting in:
introducing, into an electrodissolution cell having a soluble anode based on the said metal and an insoluble cathode without an interposed membrane, electrolyte bath to be enriched in ions of the said metal, so as to keep the said anode and the said cathode at least partially immersed,
circulating an electrodissolution electric current between the said soluble anode and the said cathode, so as to dissolve the metal in the said bath by oxidation of the said anode,
removing, from the electrodissolution cell, bath enriched in ions of the said metal constituting the solution of the said metal,
characterized in that, during these stages, an appropriate density gradient is maintained in the bath so that, if D1 is the density of the bath in the vicinity of the cathode and if D2 is the density of the bath in the vicinity of the most active part of the anode, D2 greater than D1 and (D2xe2x88x92D1)xe2x89xa7100 g/l.
The invention can also exhibit one or more of the following characteristics:
during these stages, an appropriate temperature gradient is maintained in the bath so that, if T1 is the temperature of the bath in the vicinity of the cathode and if T2 is the temperature of the bath in the vicinity of the most active part of the anode, T1 greater than T2 and the difference (T1xe2x88x92T2) is adjusted in order to maintain the said density gradient; preferably, (T1xe2x88x92T2) greater than 15xc2x0 C.
in order to carry out the process,
the bath to be enriched is introduced into the said electrodissolution cell above the level of the cathode,
the enriched bath is removed below the level of the active part of the anode,
and, the introduction and removal flow rates being approximately equal, in order to maintain an approximately constant bath level in the said cell, and defining the bath replacement flow rate,
the said replacement flow rate is adjusted in order to maintain the said density gradient.
The difference between the concentration of metal ions in the enriched bath (exiting) and that in the bath to be enriched (entering) is then taken into account.
The said metal is based on tin.
The said bath is selected from the group consisting of baths based on alkanesulphonics, on arylsulphonic, on sulphonic acid or on sulphuric acid, halide baths, fluosilicate baths and fluoborate baths.